The Joint Collaborative Team on Video Coding (jct-vc) of itu-t wp3/16 and iso/iec jtc 1/sc 29/wg 11 held its second meeting during 21-28 July, 2010 at the itu-t premises in Geneva, ch

JCTVC-B109 [E. Maani, W. Liu, A. Tabatabai (Sony)] Differential coding of intra modes (DCIM)

Spatial domain directional intra prediction has been used to reduce the correlation between the samples in the current block and the reconstructed neighbors. In AVC, 8 directional prediction modes (plus the DC prediction mode) are defined. The prediction direction is signaled to the decoder using a simple predictive coding method. The current intra prediction has two major disadvantages: 1) the small number of directions does not provide sufficient precision to cover arbitrary directional patterns; and 2) the mode number prediction from neighbors is not accurate enough to exploit the geometric dependency between blocks. Increasing the number of directions typically results in a lower residual energy, however, the cost for signaling the prediction mode may also increase significantly such that little gain is observed. This is especially the case for small block sizes such as 4x4 or 8x8. To address this problem, in this submission, we propose a new method to accurately predict the intra directions from the reconstructed neighboring pixels and differentially encode the intra directions. This allows a more precise directional prediction without significant increase in the cost for transmitting the side information. Simulation results show that the new intra prediction method can provide as much as 13% bitrate reduction compared to AVC intra prediction.

The purpose of the contribution is to have efficient signalling of prediction direction, in particular for small block sizes (previously proposed in JCTVC-A030). Direction derived by output of Sobel operator (including confidence measure). Based on the confidence measure, a delta angle is derived, such that the possible range of directions is restricted (when predictive mode is used)

Question: Does the derivation influence the latency of pipelining at the decoder side? Derivation is not instantaneous, Computation of 2x2 matrix eigenvectors is necessary for this.

Parsing is influenced by decoder (reconstruction) output, no clear separation. This could be a problem.

Differential signaling is done by unary code (1 bit flag expressing the usage of the prediction). Gain reported is 5.5%.

Was not used together with MDDT yet. There is no guarantee that current MDDT gains are additive, because it may be that the prediction error is increased .

Also not clear whether similar gain would be possible with TMuC (larger number of prediction directions etc.)

It was suggested to establish a related TE, and also investigate how this relates to the method of prediction direction prediction that is already in the TMuC; the complexity at the decoder should also be further investigated.